Dichlorodifluoromethane (CFC12), dichlorotetrafluoroethane (CFC114) and like halogenated hydrocarbons have been employed as blowing agents for producing foams of themoplastic resins and polyurethane. In resent years, however, it is pointed out that when released into the atmosphere, some of CFCs would deplete ozone in the stratospheric layer, thereby inflicting a serious adverse influence on the ecosystem including man on the earth. Consequently, a worldwide agreement calls for the restriction of use and production of CFCs having high possibility of depletion of ozone layer. From this point of view, there is a demand for development of a novel blowing agent which has no or very low risk to cause a ozone-depletion problem.
LPG seems to be employed instead of CFCs with such problems. However, using LPG as a blowing agent has another serious problem that foam products having homogeneous cells can not be obtained by LPG.